For example, Japanese Unexamined Patent Application Publication No. 2004-361279 discloses a radar sensor system in which a transmission section and a reception section for radar waves are arranged in a closed space formed between a vehicle body and a radio wave transmissive member (bumper) provided at a vehicle (paragraph [0020] and FIG. 3 of Japanese Unexamined Patent Application Publication No. 2004-361279).